Thin films of organic semiconductor material can be used in the construction of thin-film transistors (TFT). Performance of organic TFT's (OTFT), characterized by parameters such as the field effect mobility and threshold voltage, depends in part upon the molecular structure of the semiconductor film. Factors such as interfacial structure, the degree of molecular order and crystalline orientation of the thin film affect film properties.
Ordering of the semiconductor depends in turn on how the thin film is deposited. It is generally believed that increasing the amount of molecular order--by increasing crystal size, reducing the density of crystalline defects, or improving short-range molecular order-permits charge carriers, i.e. electrons or holes, to more efficiently move between molecules. This can increase the field effect mobility.
Advantageous molecular order that gives high field effect mobility can be achieved using some relatively expensive deposition techniques. In contrast, deposition techniques that enable inexpensive production or production of films of a desired uniformity and thickness can produce films that exhibit a relatively small field-effect mobility.
For example, a solvent cast film that is permitted to slowly dry often exhibits a relatively high field effect mobility when incorporated into an OTFT. Unfortunately, some deposition techniques that are more amenable to manufacturing do not readily permit slow evaporation of solvent. For example, though spin coating can yield relatively uniform thin films, the solvent usually leaves the film relatively quickly, generally leading to a low degree of crystal order. Field-effect mobility, for example, can be a factor of about 10 to 100 smaller than for cast films. Other manufacturing processes such as screen printing or various thin-film coating methods may yield desirable film morphology, but not desirable molecular order.
What are needed are lower cost manufacturing methods that produce relatively high quality organic semiconductor films.